It is known, e.g. from U.S. Pat. No. 2,159,225, to give a convex curvature to the teeth of the first carrier (referred to hereinafter as a steering nut) and to impart similar convex curvatures to the two symmetrical halves of the second tooth carrier (referred to hereinafter as a gear sector), the centers of curvatures of the two halves of the gear sector being laterally offset from the pivotal axis of that sector with a view to providing a progressively decreasing transmission ratio upon a shifting of the nut from a midposition toward either of two limiting positions. As further taught in that patent, the depth of interengagement of the teeth can be adjusted for the regulation of backlash.
A gear sector of this bilobate type, with two distinct centers of curvature, can be manufactured only with the aid of special machine tools.